MONTPELIER — Vermont lawmakers are looking to firm up a promise made by the owner of the Vermont Yankee nuclear plant that it would take extra steps to clean up the plant’s Vernon site after it shuts down.

Entergy Corp. promised when it bought Vermont Yankee in 2002 that it would take steps beyond the minimum required by the federal Nuclear Regulatory Commission to restore the Vermont Yankee site to a green field after electricity is no longer being made there.

The move comes against the backdrop of what the state says are a series of broken promises by Entergy relating to Vermont Yankee.

Chief among those was a promise that it would shut Vermont Yankee down if it had not been able to obtain a new state permit by last March.